1. Field of the Invention
The present application relates to a low noise amplifier, and apparatuses and methods for the same.
2. Description of the Related Art
FIG. 1 is a diagram schematically illustrating a general transceiver circuit. As illustrated in FIG. 1, the transceiver circuit includes a base band processor 10, a DAC (digital-to-analog converter) 12, an up-conversion mixer 14, a power amplifier 16, a duplexer 18, an antenna 20, a low noise amplifier 22, a down-conversion mixer 24, a variable gain amplifier 26, a filter 28, an ADC (analog-to-digital converter) 30 and a local oscillator 32.
In FIG. 1, since the low noise amplifier 22 is a first amplifier in a receiving path, a noise figure or noise amount of the low noise amplifier 22 is directly added to a noise figure or amount of an entire receiving path. Therefore, among the amplifiers 22 and 26 in the receiving path, the low noise amplifier 22 affects the noise figure of the entire receiving path the most. Therefore, the low noise amplifier 22 in particular should be designed to have a small noise figure.
In the low noise amplifier 22, linearity is an important factor since a single-tone desensitization and a cross modulation interference may occur because of a non-linearity. For instance, when an AMPS (Advanced Mobile Phone System) channel having a high signal level exists adjacent to a CDMA (Code Division Multiple Access) receiving channel that is a desired signal (e.g., spaced apart by 900 KHz), the AMPS channel acts as a single-tone interferer on the CDMA receiving channel. When the non-linearity exists in the low noise amplifier 22, a sensitivity for the CDMA receiving channel, which is the desired signal, is reduced by a signal of the AMPS channel (e.g., a gain is reduced). Such phenomenon is referred to as the single-tone desensitization.
In addition, when a CDMA transmitting channel signal leaks to the low noise amplifier 22 from the duplexer 18, the cross modulation interference can occur between the CDMA transmitting channel signal and the single-tone interferer. The cross modulation interference has a bandwidth corresponding to a bandwidth of a CDMA transmitting channel centered at the single-tone interferer. Therefore, the cross modulation interference can generate an interference of the CDMA receiving channel adjacent to the single-tone interferer.
To suppress the single-tone desensitization and the cross modulation interference (e.g., described above), the linearity of the low noise amplifier 22 should be improved. One easy method to improve the linearity of the amplifier is to increase a bias current. However, when the bias current is increased, a power consumption is increased, which decreases a lifespan of a battery of a mobile device. Therefore, a method for reducing or minimizing the power consumption while improving the linearity is required.
An example of the method is disclosed by a paper “Vincent W Leung, Junxiong Deng, Prasad S. Gudem, and Lawrence E. Larson, Analysis of Envelope Signal Injection for Improvement of RF Amplifier Intermodulation Distortion, IEEE Journal of Solid-State Circuits, Vol. 40, No. 9, September 2005, pp. 1888-1894” wherein the bias current is minimized when an input signal is small and the bias current is increased when the input signal is large. An amplifier and an operation thereof disclosed in the paper are shown in FIGS. 2a and 2b, respectively.
As illustrated in to FIG. 2a, the amplifier additionally includes a envelope detector 32 compared to a conventional amplifier. The envelope detector 32 detects a mean signal power and adjusts a bias current of a transistor 34 according to the detected mean signal power. When a power of an input signal increases, the bias current of the transistor is increased, whereby a load line is changed as shown in FIG. 2b. As a result, when the power of the input signal is small, an amplification is carried out using a small bias current. When the power of the input signal is large, an amplification is carried out using a large bias current. Therefore, a mean power consumption is reduced.
However, while the amplifier of FIGS. 2a-2b has a high linearity and a low power consumption, the amplifier has various disadvantageous. For example, the noise figure of the amplifier is increased. More specifically, since the output of the envelope detector 32 is connected to an input terminal of the transistor 34, a noise added by the envelope detector 32 such as a thermal noise is amplified and outputted to affect the noise figure of the amplifier. For at least such reasons, the amplifier is mainly used for a power amplifier such as power amplifier 16 where the noise figure is relatively unimportant.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.